Field of the Invention
Embodiments of the invention relate to a touch screen driver and a method for driving a touch screen.
Discussion of the Related Art
User interface (UI) is configured so that users are able to communicate with various electronic devices and thus can easily and comfortably control the electronic devices as they desire. Examples of the user interface include a keypad, a keyboard, a mouse, an on-screen display (OSD), and a remote controller having an infrared communication function or a radio frequency (RF) communication function. User interface technologies have continuously expanded to increase user's sensibility and handling convenience. The user interface has been recently developed to include touch UI, voice recognition UI, 3D UI, etc.
In recent, the touch UI has been used in portable information appliances and has been expanded to the use of home appliances. A mutual capacitive touch screen has been recently considered as an example of a touch screen for implementing the touch UI. The mutual capacitive touch screen can sense the proximity input as well as the touch input and also recognize respective multi-touch (or multi-proximity) inputs.
The mutual capacitive touch screen includes Tx lines, Rx lines crossing the Tx lines, and sensor nodes formed at crossings of the Tx lines and the Rx lines. Each of the sensor nodes has a mutual capacitance. A touch screen driver senses changes in the voltages charged to the sensor nodes before and after a touch operation and determines a touch (or proximity) position of a conductive material. To sense the voltages charged to the sensor nodes, a Tx driving circuit applies a driving pulse to the Tx lines, and an Rx driving circuit samples a small change in the voltages of the sensor nodes in synchronization with the driving pulse and performs the analog-to-digital conversion.
The Tx lines and the Rx lines of the touch screen are generally routed on a flexible printed circuit board (FPCB) and are connected to a touch integrated circuit (IC), and thus Rx channels have different RC delays. Because the driving pulse applied to the Tx lines is noisy and the Rx channels have the different RC delays, each of input signals (for example, the voltages of the sensor nodes) input to a sampling circuit has a different transition time. The transition time is a time at which the input signal changes from a transition state to a saturation state (i.e., at which the input signal is in a maximum charge state).
In FIG. 1, ‘t0’, ‘t1’, ‘t2’, and ‘t3’ denote a transition time of an Rx channel RX0, a transition time of an Rx channel RX1, a transition time of an Rx channel RX2, and a transition time of an Rx channel RX3, respectively. The transition times t0, t1, t2, and t3 are different from one another. Thus, as shown in FIG. 1, when the Rx channels RX0 to RX3 are simultaneously sampled, a sampling deviation is generated in the Rx channels RX0 to RX3. Namely, as shown in FIG. 1, when the same sampling time is applied to all of the Rx channels RX0 to RX3 using Rx sampling clocks SRx0 to SRx3, which are simultaneously generated with the same width, it is difficult to accurately detect a touch signal. Further, a signal-to-noise ratio (abbreviated to SNR) is entirely reduced because of an output noise.